Field of the Invention
The invention relates to a process for producing amplified negative resist structures.
In the fabrication of microchips, semiconductor substrates are structured using thin layers of photoresists. The chemical nature of photoresists can be altered selectively by exposure with an electron beam. The exposure can be made with the aid of a photomask or by direct irradiation. Following a developing step, in which either the exposed or the unexposed areas of the photoresist are removed, a structured photoresist is obtained which is used as a mask for the purpose, for example, of etching the semiconductor substrate. In the case of dry etching, the etching operation is completed usually with fluorine or oxygen plasma. In order to carry out selective etching of only the bare areas of the substrate, the mask-forming resist structure must possess sufficient resistance to the plasma that is used. When using an etching plasma containing oxygen, the photoresist therefore usually includes groups containing silicon. In the course of the etching operation, these groups are converted into silicon dioxide, which forms an etch-stable protective layer on the photoresist. The silicon atoms either may already be present in the photoresist polymer or may be introduced into the polymer subsequently, following the development of the resist structure, in a consolidation reaction. For this purpose, reactive groups are provided in the polymer. Examples of reactive groups include acid anhydride groups, carboxyl groups, and acidic phenolic hydroxyl groups. An amplifying agent carrying a corresponding reactive group, an amino group for example, can be chemically attached to the reactive groups.
In order to be able to realize low exposure doses and short exposure times when exposing the photoresist, photoresists known as chemically amplified resists (CARs) have been developed. In this case, the photoresist includes a photosensitive compound that on exposure liberates a catalyst. In a subsequent amplifying step, the catalyst is able to bring about a chemical reaction that changes the chemical nature of the photoresist. With a single quantum of light, which liberates one catalyst molecule, it is therefore possible to bring about a multiplicity of chemical reactions thereby achieving a marked differentiation between the exposed and unexposed areas of the photoresist. The catalyst used is usually a strong acid, which is liberated by a photoacid generator, an onium compound for example. The polymer contains acid-labile groups, such as tertiary butyl groups, which are eliminated under the action of the strong acid liberated. The elimination of the acid-labile group is generally accompanied by the liberation of an acidic group: for example, a carboxyl group or an acidic phenolic hydroxyl group. This brings about a marked change in the polarity of the polymer. The polymer originally used in the photoresist, carrying acid-labile groups, is soluble in apolar solvents or solvent mixtures having a low polarity, such as alkanes, but also in alcohols, ketones, and esters. In contrast, the polymer following elimination of the acid-labile groups is soluble in polar solvents, generally water or basic, aqueous-organic developer solutions.
In connection with the production of resist structures, a range of processes have already been developed, which can be divided into two groups according to principle.
In the case of positive photoresists, the exposed areas of the photoresist are detached in the developing step and in the structured photoresist, for example, form the trenches, whereas the unexposed areas remain on the substrate and form, so to speak, the lines of the photoresist structure.
For producing positive photoresist structures, the procedure described above can be followed. The exposure initiates a chemical reaction within the photoresist. The chemical reaction causes the photoresist polymer to become soluble in alkaline developer solutions: for example, a 2.38% strength solution of tetramethylammonium hydroxide in water. Then, development creates a corresponding positively structured photoresist.
In the case of negative resists, in contrast to the positive-working resists, the exposed portion of the resist remains on the substrate whereas the unexposed portion is removed by the developer solution. When working with chemically amplified negative resists, exposure initially likewise liberates a catalyst, usually a strong acid. The catalyst facilitates crosslinking in the photoresist. As a result, the solubility of the polymer in the developer medium is reduced. The crosslinking causes the exposed area to become insoluble, whereas the unexposed area can be removed in appropriate developers. Developers used are generally aqueous solutions, so that the polymer usually has polar groups in the unexposed state.
For a modification of the developing step, a positive photoresist can also be used to produce a negative resist structure. A process of this kind is described, for example, in U.S. Pat. No. 4,491,628 issued to Ito et al. Ito et al. teach exposing a layer of a positive photoresist attached to a substrate. In this step, an acid is liberated from a photoacid generator. In the subsequent amplifying step, the acid-labile groups in the exposed areas are eliminated by heating, so the polymer is changed to a polar form. In contradistinction to the positive developing process described above, the development is then conducted not with a polar aqueous developer but instead with an apolar solvent. As a result, only the unexposed areas of the substrate, in which the polymer has retained its original apolar form, are detached. Because the polar fractions of the resist, in which polar groups—carboxylic acid groups, for example—have been produced by the exposure, are insoluble in apolar solvents, they remain as lines on the substrate.
Another negative photoresist includes not only a photobase but also a thermoacid. A resist of this kind is described, for example, in International Application No. PCT/DE00/04237. On exposure of the photoresist, a base is liberated in the exposed areas. In a subsequent amplifying step, an acid is liberated from the thermoacid generator by heating. In the exposed areas the acid is neutralized by the base liberated beforehand and is therefore no longer available as a catalyst. In the unexposed areas, the acid catalyses the elimination of the acid-labile groups from the polymer. Accordingly, in the unexposed areas, the polymer is converted from its apolar form into a polar form. In the subsequent developing step, therefore, the unexposed areas can be selectively detached from the substrate using an aqueous-alkaline developer, while the exposed areas remain, so to speak, as lines on the substrate.
As already mentioned, for the etching of the substrate the resist structure must possess sufficient etch resistance. For this purpose, for instance, the lines of the resist structure must have a sufficient layer thickness. This is a particular problem in the case of resists for the 157 nm and the 13 nm technology, because at these wavelengths the photoresists known to date exhibit high absorption. Accordingly, only very thin polymer films can be used, in order to ensure that the radiation used for exposure is able to penetrate even into the deep areas of the resist in sufficient intensity, and liberates sufficient quantities of catalyst. If insufficient quantities of catalyst are liberated in the lower layers of the photoresist, elimination of the acid-labile groups is incomplete or, in a worst-case scenario, does not take place at all. A consequence of this is that following development, residues of the polymer remain in the trenches, forming what are known as resist feet. Owing to its low layer thickness, the resistance of the structured photoresist to an etching plasma is insufficient, which is why its etch resistance must be increased. For thin purpose, following development, the structured resist is chemically amplified. Where the resist structures have a sufficient layer thickness, it is also possible, in addition to an increase in layer thickness, to bring about a narrowing of the trenches, perpendicularly to the substrate surface, by laterally growing layers on the sidewalls of the trenches of the structured resist. As a result, it is possible to achieve an improvement in resolution: that is, for example, the reproduction of narrower conductor tracks. A process of this kind is described, for example, in European Patent EP 0 395 917 B1, which corresponds to U.S. Pat. Nos. 5,234,794 and 5,234,793. In order to amplify the resist structure, the amplifying agent, in solution in a suitable solvent or else from the gas phase, can be applied to the structured resist. The incorporation of silicon-containing amplifying agents into the polymer is generally referred to as silylation.
In fluorine plasma, volatile silicon tetrafluoride is formed from the silicon present in the resist. In this case, amplifying the structured resist with silicon atoms makes no sense. In order to raise the resistance of the resist toward a fluorine plasma, the structured resist is amplified using aromatic amplifying agents. This amplification is referred to as aromatization.
In order to transfer structures produced with very short wave length exposing radiation into a substrate, a resist system including two layers has been used to date. The top layer of the resist system is comparatively thin and photostructurable. Following exposure, contrasting, and developing, the structured resist is amplified with a silicon-containing amplifying agent and the structure is transferred into the bottom layer of the resist system using an oxygen plasma. The bottom layer is composed, for example, of a resist which, although having a low etch resistance toward an oxygen plasma, possesses a high etch resistance toward a fluorine plasma. A resist of this kind includes polymers having a high aromatic fraction, an example being an etch-resistant novolac, a cresol resin. After the structure of the top resist layer has been transferred into the bottom resist layer with an oxygen plasma, the plasma is changed and the structure is transferred into the substrate using a fluorine plasma. The substrate in this case is composed, for example, of silicon, silicon nitride, or silicon dioxide, so that the material of the substrate can be ablated by the conversion of the silicon, containing substrate into volatile silicon tetrafluoride. Owing to the two-layer resist, the process is relatively expensive and technically complicated by the change of plasma system.
The existing processes for producing amplified resist structures involve a multiplicity of worksteps and are therefore very complicated to conduct. Every workstep also increases the error rate in the fabrication of microchips, meaning that a correspondingly high reject rate must be tolerated. This is also a problem on account of the fact that nondestructive testing is not possible at every step in microchip fabrication. Generally, error testing of this kind is possible only after several production steps, since it is only then that the electrical connections necessary for testing are present in the microchip. In some circumstances, therefore, several weeks may pass between a production step and error testing. Accordingly, an extremely low error rate is required for each production step.
Additionally, the chemical consolidation requires corresponding reactive “anchor” groups in the resist polymer, to which the amplifying agent can be attached. Preparation of these polymers necessitates processes that are likewise complex, since, for example, they must be carried out in the absence of moisture in order to prevent premature hydrolysis of the reactive anchor groups.